This invention relates to methods and circuits for controlling the frequency of clock signals and, more particularly, to such methods and circuits which are used to control the output frequency of an electronic inverter.
Constant speed drive electrical power systems which are typically found on commercial aircraft, couple an electric generator to the aircraft engine through a hydromechanical transmission which drives the generator at a constant speed to produce constant frequency output voltage. Variable speed constant frequency (VSCF) power systems include a generator which is directly coupled to the engine and therefore driven at variable speeds. The variable frequency output of this generator is electronically converted to a constant frequency output. It is desirable to retrofit existing constant speed drive equipped aircraft with variable speed constant frequency power systems. A key to successful retrofit lies in the design of a VSCF system which is directly interchangeable with existing constant speed drive systems. This precludes aircraft wiring changes or changes in any of the other system components. To accomplish this objective, the VSCF converter and its controls must be in the same package as the generator.
In a typical aircraft electrical system in which a VSCF system has replaced a constant speed drive system, the VSCF system is mounted on the engine and used to supply three phase AC power via feeder conductors to a remote load. A line contactor, located near the load, is used to disconnect the load from the feeder conductors. A remote generator control unit can be used to provide the required control of this contactor.
Inverter circuits such as those found in DC link VSCF power systems include a plurality of power pole switching elements which operate with a fixed switching pattern generated by a microprocessor or other digital circuit. A crystal oscillator is used as the frequency reference to control the output frequency of the inverter to within, for example, less than 0.05% of 400 Hz. Since the remote generator control unit typically will include underfrequency protection circuits, these circuits can be used to advantage to control the operation of the line contactor which connects the remote load to the feeder conductors. This may be accomplished by lowering the VSCF system frequency when it is necessary to open the line contactor. When the previously existing frequency sensing circuit in the remote generator control unit senses an underfrequency, it will command the line contactor to open. One technique of implementing this type of line contactor control would be to cause a step change in the output frequency of the inverter, for example, from 400 Hz to 359 Hz. However, if the VSCF system is supplying motor loads, this may result in an undesirable voltage transient. It is therefore, desirable to provide a circuit for lowering the output frequency of the inverter by ramping the frequency between two previously determined frequencies. A circuit which may be used to perform this function is disclosed in U.S. Pat. No. 4,618,920, issued Oct. 21, 1986. The circuit of that patent uses a voltage controlled oscillator to remove pulses from a high frequency clock signal in response to a control voltage which may be provided by a ramp generator. Using that technique, the low frequency point is dependent upon the accuracy and stability of the voltage controlled oscillator and its timing components. Additionally, the number of components required to implement this approach, adds to the complexity of the clock circuit.
It is therefore desirable to devise a frequency control technique which provides the required accuracy without the need for additional circuit components.